As well known, there are systems for used in various production lines incorporating weighing/sorting machines (so called auto-checker) for automatically weighing objects (works) while they are being conveyed and sorting them depending on the weight data obtained for the objects.
Some of the basic requirements for such systems include high speed, enhanced accuracy and improved reliability.
A weighing/sorting machine of the type as described above normally comprises as principal components a number of conveyors including a feeding conveyor, a weighing conveyor and a sorting conveyor. These conveyors are required to have a reduced size, durability, a high torque and stability particularly in terms of the conveyor drive/control system including the motor in order to meet the above requirements for the machine.
It is a recognized fact that any conveyors of conventional weighing/sorting machines do not satisfy the above requirements.
Some of the reasons for this are discussed below.
FIG. 36A of the accompanying drawings illustrates the configuration of a conventional weighing/sorting machine, where reference numeral 1 denotes a feeding conveyor driven by motor M1 at a given speed to move the objects placed on it at a required rate under the control of a control unit 2.
Reference numeral 3 denotes a weighing conveyor for weighing one by one the objects W fed in by the feeding conveyor 1 while they are moved further on, said conveyor being also driven by a given speed by motor M2 to move the objects on it at a required rate under the control of a control unit 5.
Reference numeral 6 denotes a sensor for detecting object W being transferred to the weighing conveyor 3 and reference numeral 7 denotes a sorter circuit for assigning the object W to a specific group of objects by transmitting a sorting signal after the elapse of a given period of time since the object is completely transferred to the weighing conveyor 3 or a period of time required to obtain a stabilized weight signal for the object.
Reference numeral 8 denotes a sorting conveyor for assigning the object W coming from the weighing conveyor a specific route corresponding to the sorting signal given to it and causing it to be moved away by a guide device 9 which is operated in accordance with the sorting signal. For instance, the weighed objects W may be divided into three different groups and collected at the ends of three different routes.
Like the two other conveyors, the sorting conveyor 8 is driven by motor M3 at a given speed to moving the objects on it at a required rate under the control of a control unit 10.
The three motors M1, M2 and M3 to be used for diving the respective conveyors are single-phase induction-type brushless motors that can be realized in small dimensions and withstand continuous operation for a prolonged period of time. The rotary speed of the motors M1, M2 and M3 can be modified by changing the level of the voltage of power applied to them by means of the respective control units 2, 5 and 10.
The three conveyors are normally operated at a same speed with a view to minimize the vibration that may be generated in the machine to adversely affect the operation of the weighing conveyor 3 as well as the time required for it to receive an object and weigh it.
Therefore, the object which is brought to the feeding conveyor 1 is moved at a predetermined constant speed to the weighing conveyor, weighed there and then sorted by weight while it is moved away from the location of weighing.
The object transferred from the weighing conveyor 3 to the sorting conveyor 8 is guided to its proper take-out position by the sorting signal generated for it and then further moved away to the next stage of the production line.
However, a weighing/sorting machine of the type as described above comprising a plurality of conveyors that need to be so controlled as to operate at a same speed is accompanied by a problem that all the conveyors have to be adjusted for speed whenever the machine has to deal with objects of a type which is different from the previously dealt ones.
As a solution to this problem, an identical control signal may be given to each of the control units 2, 5 and 10 in order to collectively control the moving speed of the conveyors by applying an identical voltage to all the motors M1, M2 and M3. But, such a solution is not at all effective when the motors have different operational features including the transmission gear ratios or when the loads applied to the entire machine by the conveyors are significantly different from one another.
A control unit of the type as described above for controlling the speed of operation of the conveyors by varying the voltages applied to the drive motors normally requires elaborate efforts to finely control the speed. Otherwise the motors would not be operated at an exactly same and identical speed.
The rate of sorting and hence the speed of the conveyors of a weighing/sorting machine of the type as described above have to be reduced if it has a long distance for the objects to be conveyed for sorting. However, a weighing/sorting machine whose speed of the conveyors is controlled by varying the voltage applied to them is accompanied by a problem of reduced torque of each of the motors M1, M2 and M3 when the voltage applied to it is reduced to lower the speed of the conveyor for which it is responsible, a problem that arises when sorting heavy articles.
Now, electric motors are used not only for conveyors of the type as described above but also for those installed in production lines for purposes other than weighing/sorting, for instance for quality examination.
Currently, single-phase induction motors dominate these applications because they do not involve brush friction and heat generation as DC motors do.
FIG. 36B of the accompanying drawings shows the configuration of a conventional control unit for controlling a single-phase induction motor.
This single-phase induction motor 11 (hereinafter referred to simply as motor) is so designed as to rotate rotor 12 by applying two AC voltages with a same level but phases differentiated by 90 degrees respectively to first and second coils L1 and L2 (C0 donating a capacitor for phase differentiation) so that generator 13 generates AC signals in synchronism with the rotation of the rotor
Motor control unit 110 converts the AC signal from the generator 13 of the motor 11 into a DC voltage signal by means of a rectifier circuit 111 and compares the voltage with a control voltage transmitted from a variable resistor 112 for speed control by using a comparison/control circuit 113.
The signal representing the result of the comparison is sent to a thyristor 114 arranged between the commercial power source 115 and the motor 11 to adjust the voltage of the AC power source 115 so that the two voltages applied to the comparison/control circuit 113 become equal relative to each other.
Thus, the voltage of the commercial power source is increased by raising the level of the control voltage through the use of the variable resistor 112 for setting the motor speed to augment the rate of rotation of the motor 11. Conversely, the voltage of the commercial power source is decreased by reducing the level of the control voltage through the use of the variable resistor 112 to lower the rate of rotation of the motor 11. In this way, the motor speed can be appropriately controlled.
However, with such an arrangement of the control unit of the motor, since the voltage of the commercial power source has to be lowered to reduce the torque of the motor, is accordingly reduced whenever a low rate of rotation of the motor 11 is needed, the motor cannot exert a high torque at low speed and the conveyor driven by the motor inevitably falls short of the power it needs.
Besides, because of the fact that the motor speed is controlled by means of the AC voltage of the generator 13, whose output does not linearly change, the motor speed itself is subject to fluctuations. This brings forth a serious problem particularly when more than two motors are to be controlled to run at an identical speed.
It should be noted that this is a fatal and inescapable problem for a so-called speed-controlled (variable speed ) motor.
While the use of a three-phase motor may provide a solution for this problem, it is accompanied by a problem of bulkiness when compared with a single-phase 100 V motor as a three-phase motor is normally designed for use with a 200 V commercial power source.
Moreover, a DC motor does not meet the requirement of durability because a brush is always used there.
As discussed above, whenever an induction motor is controlled for its rotary speed by varying the voltage of the commercial power source used for it, the torque of the motor is always subject to change and, therefore, the rate of rotation of the motor cannot be varied to a considerable extent without causing the torque problem.
FIG. 36C shows how a technique of converting the electricity of the commercial power source 201 into AC having a desired frequency by means of an inverter 203 is used before the power is applied to a load circuit 202 including an induction motor in order to bypass this problem.
The inverter 203 comprises a rectifier circuit 204 for converting the AC voltage of the commercial power source into the DC voltage, a modulator circuit 205 for modulating the DC voltage by a given frequency (pulse width and pulse number modulation) and a frequency controller 206 for generating a frequency specifying signal so that the frequency of the power source may be modified within a given range (several Herz to several hundreds Herz) before the power from the power source is applied to the load circuit 202.
If, however, the frequency of the power source is carelessly reduced by means of an inverter before the power is applied to the inductive load circuit including an AC motor, the level of the electric current running through the load circuit is remarkably raised to give rise to an increased possibility of accidental fire due to a heated and scorched coil.
An inverter used to drive a motor is provided with a frequency controller for variably specifying a frequency for the power source to determine the rate of rotation of the motor.
FIG. 36D is a block diagram of a conventional variable frequency type inverter 301.
In the block diagram, reference numeral 302 denotes a frequency controller that converts the level of an output voltage of potentiometer 303 into a corresponding digital value by means of an AD converter 304.
Also, reference numeral 305 denotes an AC conversion circuit for converting the AC from the commercial power source into an AC having a frequency corresponding to the signal transmitted from the AD converter 304 so that the AC from the commercial power source is once converted into DC, which is then further converted into AC by means of a switching circuit 307 that continuously switches the polarity of the DC with the specified frequency in order to drive a load circuit 309 with AC power.
Reference numeral 308 denotes a switching control circuit for controlling the switching circuit 307 according to the digital value transmitted from the AD converter 304. For instance, it sends out to the switching circuit 307 a signal having a pulse width that varies to form a sinusoidal wave at a frequency corresponding to the digital value in order to continuously switch the polarity of the DC and to drive the load circuit 309 by AC.
With such an arrangement, the AC power applied to the load circuit 309 be made to vary according to the output voltage of the potentiometer 303 of the frequency controller 302, which is externally controllable.
However, an inverter as described above for controlling the frequency of the power applied to the load circuit by controlling the voltage does not operate satisfactorily with a desired level of accuracy because variations that appear in the voltage and the temperature are considerable.
Particularly when more than two load circuits are synchronously driven by more than two inverters, tremendous efforts are normally required for synchronization of the involved frequencies.
Additionally, as the AC motor to be driven by an inverter is a three-phase motor, the inverter should comprise as a matter of course a three-phase motor driving inverter control system. These and other problems have so far considerably hindered the use of three-phase motor driving control systems comprising inverters for weighing/sorting machines as described earlier practically unfeasible.
Modern production lines normally incorporate inspection systems for checking the weights of works and foreign objects mingled or mixed with the works somewhere on the conveyors they comprises.
FIG. 31 is a schematic illustration showing part of a production line incorporating such an inspection system.
In FIG. 31, reference numeral 401 denotes a feeding conveyor to be used for the inspection system.
Objects of inspection W being moved on the feeding conveyor 401 at a given constant speed with an identical spatial interval between any two successive objects are sequentially brought to an inspection conveyor 403 by way of an intermediate conveyor 402 and the objects that have been examined on the inspection conveyor are forwarded one by one to the next stage of the production line.
The inspection conveyor 403 has a length sufficient enough to carry out a weight check or a check for detecting foreign objects. If the feeding conveyor 401 bring in objects of inspection with an interval which is shorter than the length of the inspection conveyor 403, it means that two or more than two objects are found simultaneously on the feeding conveyor 401 and therefore the inspection can entail erroneous results.
Conventionally, this problem is avoided by driving both the intermediate conveyor 402 and the inspection conveyor 403 at a speed V2 faster than the speed V1 of the feeding conveyor 401 to provide a large interval between two successive objects of inspection on the inspection conveyor and by equalizing the speed of the intermediate conveyor 402 and the inspection conveyor 403 with a view to minimize the shock that might be given to the inspection conveyor 403 by the intermediate conveyor 402 when an object is transferred from the latter to the former.
With such an arrangement, however, the object being transferred from the feeding conveyor 401 moving at speed V1 to the intermediate conveyor 402 running at speed V2 is inevitably subjected to an abrupt and large acceleration, causing any unstable objects of inspection to fall and, in some cases, the contents of the objects to be undesirably biased.
Moreover, if the objects are realized in the form of a round rod and apt to roll on the conveyor, they cannot be put on the conveyors perpendicular to the direction of movement, making the interval between two successive objects inevitably large to reduce the efficiency of inspection.
In short, any conventional conveyors used for weighing/sorting machines fall short of satisfying the requirements of reduced dimensions, durability, high torque, elevated stability and so on called for conveyor drive control systems using AC motors as sources of motive power.
Consequently, a highly efficient, highly accurate and highly reliable weighing/sorting machine or system cannot be established by using such conventional conveyors.